Spam filter
Software at the receiving side that classifies incoming mail as wanted, spam or worse, then accepts, rejects, quarantines or files it in the spam folder.
What it means
Filters combine several layers. Microsoft's documentation describes incoming mail passing connection filtering first, which checks the sender's reputation and rejects most spam, then malware scanning, then spam filtering that assigns verdicts such as spam, high confidence spam, phishing, high confidence phishing and bulk. Apache SpamAssassin, an open-source filter, scores messages with heuristic and statistical tests on headers and body, including Bayesian filtering, DNS blocklists and collaborative filtering databases.
Filters also learn from users: Microsoft says its filtering learns from user feedback on Outlook.com, and Gmail says it recognises similar mail better as users report spam.
Senders cannot see a filter's decision directly. They infer it from SMTP rejections, provider dashboards, placement tests and headers such as Microsoft's X-Forefront-Antispam-Report.
Related terms
On this site
Sources
What this definition rests on. The wording is ours; the facts are theirs.
- Microsoft Learn: Built-in security features for all cloud mailboxes (EOP)learn.microsoft.com
- Microsoft Learn: Anti-spam protectionlearn.microsoft.com
- Apache SpamAssassinspamassassin.apache.org
- Gmail Help: Report spam in Gmailsupport.google.com
- Microsoft Learn: Spam confidence level (SCL)learn.microsoft.com
All 432 terms in the glossaryThe wiki: blocklists and bounce codes